Protective coating for magnesium



Patented Feb. 23,1926.

UNITED STATES LEWIS J". KEELER, OF NIAGARA PATENT OFFICE.

FALLS, NEW YORK, ASSIGNOR T0 AMERICAN MAG- NESIUM CORPORATION, OF NIAGARA FALLS, NEW YORK, A CORPORATION OF NEW YORK.

PROTECTIVE COATING FOR MAGNESIUM.

No Drawing.

To all whom it may concern:

Be it known that I, LEWIS J. KEELER, citizen of the United States, residing at .Niagara Falls, in the county of Niagara and State of New York, have invented certain new and useful Improvements in Protective Coatings for Magnesium, of which the following is a specification.

My invention relates to protective coatings for magnesium and magnesium base alloys. Various shades of color coatings from a light golden to a deep jet black may be obtamed by my process. The coating is of a very bright luster, of measurable depth, very dense and will withstand severe handling.

The coating will withstand severe atmos pheric influences.

and magnesium alloys, when treated by my process, show a marked improvement in their Articles of magnesium stability as compared with articles not so treated, when subjected to weak acids or salt solutions.

For carrying out my process, I use a solution or an electrolyte which may be composed of the bichromate or normal chromate salts of potassium or sodium acidified with any suitable acids. The.compo 'sition of the solution or electrolyte may vary considerably and in special cases the acid may be omitted and the process carried out in a neutral solution. Too great an acidity is to be avoided for this tends to make a'coating which will not be adherent. The solution which I prefer to use in coating ma nesium and magnesium base alloys is ma e as follows: acid, specific gravity 1.84, is added to 6000 cubic centimeters of a solution in the proortion of three, arts by weight of sodium ichromate and 9 parts by weight of water. The metal to be coated is cleaned in dilute nitric acid and thereafter washed free from acid in running water. It is then made the anode of an electrolytic cell in which the electrolyte is the bichromate solution described above. The cathode may be any suitable electrical conductor. I have found magnesium metal or carbon to be best adapted for this purpose.

The current employed is preferably direct current although alternating current can be used. In order to-secure a firmly adherent coating, the anode current density should be low, such .as .0005 ampere per square inch. It can vary below this but when a higher 1 cubic centimeter sulphuric Application filed January 22, 1923. Serial No. 614,305.

current density is employed, the deposit does not adhere as firmly. The metal while being coated is preferably agitated to insure a goodcoating. The time in which the coating will be secured depends upon the depth of color desired. At first a light golden color will be obtained which w1ll gradually grow darker until it reaches a jet black. WVhen the desired color is obtained the article is removed from the cell,

washed with running water and dried. It'

does not need to be polished as it has acquired a brilliant luster during the course of deposition.-

Magnesium has the property of precipitating chromium from its neutral or slightly acid solutions in the form of oxides. The coating secured by my process is a complex mixture of chrom um and magnesium oxides and hydroxides. Agin of the freshly prepared coating tends to c ange the hydroxide to the oxide thereby yielding a coating of a more resistant nature.

Having described the various phenomena which take place in my process, I will now outline the details of procedure in coating a magnesium or ma esium alloy rod. The rod is first cleane by dipping in dilute nitric acid for approximately one half minute. The acid solution adhering to the rod is now quickly washed off in running water and it is at once submerged in the electrolyte described above. The positive terminal of a direct current circuit is attached to the rod and the current regulated to an anode density of .0005 amperes per square inch. The rod remains in the cell until the desired color is obtained. A jet black color is obtained in approximately 10 minutes. A

vlight golden color is obtained in about one half this time. After the desired color has been obtained, the rod is removed from the cell, washed and allowed to dry. The coating upondrying acquires a very high luster without further treatment.

I claim: p

1. The process of coating magnesium and its alloys which comprises electrolyzing a solution of an alkali metal chromate, using the said metal as an anode. I

"2. The process of coating magnesium and its alloys which comprises electrolyzing a solution of an alkali metal chromate, using the said metal as an anode and-maintaining a low current density.

3. The process of coating magnesium and its alloys which comprises electrolyzing a solution of an alkali metal chromate, using the said metal as an anode and maintaining a current density below .0005 ampere per square inch.

4. The process of coating magnesiumand its alloys which comprises electrolyzing an acid solution of sodium bichromate using the said metal as an anode.

5. The process of coating magnesium and its alloys which comprises electrolyzing an acid solution of an alkali metal chromate, using the said metal as an anode.

6. A magnesium or magnesium alloy article having on its surface an adherent coating of electro-deposited chromium-magnesium oxides.

7. The process of coating magnesium and its alloys which comprises submitting the metal to anodic oxidation in an electrolyte containing an oxidizing agent in solution.

In testimony whereof I aflix my signature.

LEWIS J. KEELER. 

